Optical symbology readers detect and identify optical signals reflected from a symbology on a target object. To perform this function, symbology readers usually require an illuminating light source to illuminate the target object. An area-type symbology reader simultaneously receives and detects light from a large area of the target object which encompasses a significant portion of a symbology. For adequate illumination a wide field illumination source or a plurality of sources is used, usually controlled by flash optics. That is, an entire region of the target object containing the symbology is illuminated for a relatively short period of time.
To detect light from a large area, detector assemblies including imaging optics and detector arrays, such as commercially available CCD arrays having a plurality of light detecting elements, are used. Each element produces an electrical signal in response to the amount of reflected light received from a portion of the illuminated target object. With a conventional bar code, a region of the symbology with low reflectance is read as a black bar and a region with high reflectance is read as white space around a bar.
A significant problem with detector assemblies is that incident light energy can be sufficiently high that all areas of the target object are reflecting the maximum readable light level of the detector assembly for a given gain. The problem is encountered when the target object has a glossy surface which reflects a substantial portion of the illumination light toward the detector assembly, even from those regions which are intended to be of relatively low reflectance. Such a surface is typically perceived as a glossy surface, and such reflection is referred to as specular reflection. If the label is glossy, even a black region of a symbology may reflect light from the illuminating light source with relatively high efficiency if the illuminating light source and detector assembly are at certain angles with respect to the symbology. This problem may be solved by reducing the gain of the detector assembly; however, such a reduction in gain will lower the signal from non-specular areas and make the nonspecular signal difficult to detect. This also increases the complexity of the reader by requiring a gain control mechanism and causes delays due to the response time of the gain control system.
A further problem with reducing the gain of the detector assembly occurs where the specular reflection due to "glossiness" of the target object is localized. That is, in some cases, the specular reflection may occur in only relatively small portions of the illuminated surface. In such situations, to solve the problem by adjusting gain, the gain of the entire detection system must be reduced to prevent a portion of the array from being saturated by the reflected light from small portions of the illuminated surface. In typical arrays, gain may not be selectively reduced only in selected portions of the arrays. Thus, in those areas where specular reflection is not present, the reduced gain with its associated reduced sensitivity may make detection of a symbology difficult.
Even adjusting gain in selected portions of the detector assembly where specularly reflected light is incident does not assure a symbology will be read correctly. The specularly reflected light may still cause the detector assembly to perceive the portion of the symbology where specular reflection occurs as a high reflectance region, where, in reality, the region is of low reflectance in a non-specular sense. Therefore, specular reflection can "wash out" the information from areas of the image. Even if the gain is selectively controlled for different portions of the array, the ability of the detector assembly to identify low reflectance regions of the symbology is impaired.
A "glossy" reflection may be reduced somewhat by providing a matte finish to the symbology; however, this typically does not eliminate the problem entirely. Further, such a finish imposes a requirement on symbologies which may be difficult to meet. For example, in thermally printed bar codes on common printing stock, a certain amount of glossiness typically results. Using a rougher printing stock to reduce reflection often degrades the performance of thermal printers and blurs edges between successive regions of high and low reflectance. As is known in the art, blurred edges reduce the accuracy with which the symbology may be detected, making this approach undesirable.
Specular reflection may also be reduced somewhat by tilting the illuminated surface or illumination source beyond the angle of specular reflection. This makes the scanning process more difficult expensive and time consuming.